This invention relates generally to radio frequency circuits and more particular, to radio frequency phase shifters.
As is known in the art, phase shifters are used in a variety of applications to provide a selected phase shift to a signal propagating therethrough. One of the more frequent applications of phase shifters are in transceiver modules (transmit/receive modules) of phased array antennas. Certain applications of phased array antennas require high phase resolution, typically about 0.7.degree. in phase.
One approach to provide high phase resolution has been to cascade multiple sections of digital phase shifters. With a digital phase shifter, each section is selected to provide a differential phase shift bit corresponding to 180.degree./2.sup.n, where n is an integer between 1 and i, with i being the last phase shift bit. To provide a phase resolution of 0.7.degree. requires 9 bits or nine cascaded phase shifter sections. This approach is in general not practical because of the increase in insertion loss, noise, and VSWR. Moreover, a chain of nine phase shifter sections would make the phase shifter relatively large and also costly.
An alternative approach in the prior art is to cascade a continuously variable phase shifter with a 3-bit digital phase shifter. The 3-bit digital phase shifter provides 45.degree. of resolution and the variable phase shifter provides a continuous variable phase shift over the range of 0.degree.-45.degree.. Several approaches are known to provide an analog phase shifter having a continually variable phase shift over 0.degree.-45.degree.. Generally such approaches require the use a 90.degree. hybrid. The requirement of a 90.degree. hybrid provides a phase shifter which is physically relatively large and also difficult to fabricate. Such hybrids also have restricted bandwidths which generally are more restricted than the requirement for many applications of such phase shifter circuits. Other approaches which provide variable phase shifters use active devices such as field effect transistors. However, these approaches are non-reciprocal, and it is generally desired in most applications of phase shifters particularly in a phased array that the phase shifters be reciprocal so that the same wavefront can be received on echo as is transmitted from the array without having to change the state of the phase shifters or without requiring extra circuitry to change the direction of signal transmission.
Accordingly, there exist a need for a compact, reciprocal continually variable phase shifter.
It is also known that T/R modules is a phase array often number in the hundreds or thousands. Generally, all of said modules are designed to be identical and to have identical phase delays between a pair of input/output ports thereof. It is generally observed that due to inherent tolerances in manufacturing of said modules that the phase delays between the pair of ports is not identical. Thus, an arrangement is needed to change the pathlength without substantially increasing the size or power consumption of the module.